尝试在JavaFX中切换场景时出现NullPointerException

Question

我试图点击GUI中的按钮并切换到同一个舞台中的单独场景。这看起来很简单，只是当我单击按钮时，我在Controller类中得到NullPointerException。该按钮使用fx:id="settings"和onAction="#handleSettings"以FXML编写。 handleSettings方法位于控制器中。

我试图通过在顶部写primaryStage来将Main.java中的public Stage primaryStage;导入到Controller.java中，但我显然遗漏了一些东西。 GUI启动正常，但单击“设置”按钮时会发生错误。完整代码如下以及错误。该错误指向此行：primaryStage.setScene(sceneSettings);

我是否需要以某种方式使用get / set for primaryStage？显然，我在技能水平上处于第1级。什么都有帮助，谢谢。

Main.java

import ...

public class Main extends Application {
          public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage primaryStage) throws Exception{
        Parent root = FXMLLoader.load(getClass().getResource("sample.fxml"));
        Scene scene = new Scene(root, 450, 580);
        scene.getStylesheets().add("styleMain.css");
        primaryStage.initStyle(StageStyle.UTILITY);
        primaryStage.setTitle("Scene 1");
        primaryStage.setScene(scene);
        primaryStage.show();
    }
}

Controller.java

import ...

public class Controller{
    Scene sceneSettings;
    public Stage primaryStage;

    public void handleFieldData(){
        System.out.println("field data");
    }

    public void handleComments(){
        System.out.println("comments");
    }

    @FXML
    public void handleSettings() throws IOException {

        Button button2 = new Button("settings");
        StackPane layout = new StackPane();
        layout.getChildren().add(button2);
        sceneSettings = new Scene(layout, 450, 580);
        primaryStage.setScene(sceneSettings);
        primaryStage.setTitle("Settings Menu");
        primaryStage.show();
    }
}

sample.fxml

<GridPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/8.0.40" xmlns:fx="http://javafx.com/fxml/1"
          fx:controller="Controller">

      <columnConstraints>
    <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="100.0" />
    <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="100.0" />
  </columnConstraints>
  <rowConstraints>
    <RowConstraints minHeight="10.0" prefHeight="20.0" vgrow="SOMETIMES" />
    <RowConstraints minHeight="10.0" prefHeight="20.0" vgrow="SOMETIMES" />
  </rowConstraints>
   <padding>
      <Insets bottom="10.0" left="70.0" right="70.0" top="10.0" />
   </padding>
   <children>
      <Button fx:id="comments" mnemonicParsing="false" onAction="#handleComments" GridPane.columnIndex="1" GridPane.halignment="CENTER" GridPane.valignment="CENTER">
         <padding>
             <Insets bottom="15.0" left="50.0" right="50.0" top="90.0" />
         </padding>
      </Button>
       <Button fx:id="settings" mnemonicParsing="false" onAction="#handleSettings" text="Settings" GridPane.columnIndex="1" GridPane.halignment="CENTER" GridPane.rowIndex="1" GridPane.valignment="CENTER">
           <padding>
               <Insets bottom="15.0" left="30.0" right="30.0" top="15.0" />
           </padding>
       </Button>
      <Label text="Comments" GridPane.columnIndex="1" GridPane.halignment="CENTER" GridPane.valignment="TOP" />
   </children>
</GridPane>

错误：

Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1774)
    at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1657)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
    at javafx.event.Event.fireEvent(Event.java:198)
    at javafx.scene.Node.fireEvent(Node.java:8411)
    at javafx.scene.control.Button.fire(Button.java:185)
    at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:182)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:96)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:89)
    at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
    at javafx.event.Event.fireEvent(Event.java:198)
    at javafx.scene.Scene$MouseHandler.process(Scene.java:3757)
    at javafx.scene.Scene$MouseHandler.access$1500(Scene.java:3485)
    at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1762)
    at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2494)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:352)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:275)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$355(GlassViewEventHandler.java:388)
    at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:387)
    at com.sun.glass.ui.View.handleMouseEvent(View.java:555)
    at com.sun.glass.ui.View.notifyMouse(View.java:937)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$149(WinApplication.java:191)
    at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:71)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:275)
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1771)
    ... 48 more
Caused by: java.lang.NullPointerException
    at Controller.handleSettings(Controller.java:38)
    ... 58 more

Answer 1

加载控制器时，需要将其传递给初级阶段。

public class Controller{
    Scene sceneSettings;
    public Stage primaryStage;

...
public void setPrimaryStage(Stage stage) {
  this.primaryStage = stage;
}
...

主要

 @Override
    public void start(Stage primaryStage) throws Exception{
        FXMLLoader loader = new FXMLLoader();
        loader.setLocation(getClass().getResource("sample.fxml"));
        Parent root = loader.load();
        loader.getController().setPrimaryStage(primaryStage);

        Scene scene = new Scene(root, 450, 580);
        scene.getStylesheets().add("styleMain.css");
        primaryStage.initStyle(StageStyle.UTILITY);
        primaryStage.setTitle("Scene 1");
        primaryStage.setScene(scene);
        primaryStage.show();
  ...